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[PATCH v4 4/4] x86/time: use fake read_tsc()
- To: "xen-devel@xxxxxxxxxxxxxxxxxxxx" <xen-devel@xxxxxxxxxxxxxxxxxxxx>
- From: Jan Beulich <jbeulich@xxxxxxxx>
- Date: Thu, 31 Mar 2022 11:31:38 +0200
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- Cc: Andrew Cooper <andrew.cooper3@xxxxxxxxxx>, Wei Liu <wl@xxxxxxx>, Roger Pau Monné <roger.pau@xxxxxxxxxx>
- Delivery-date: Thu, 31 Mar 2022 09:31:46 +0000
- List-id: Xen developer discussion <xen-devel.lists.xenproject.org>
Go a step further than bed9ae54df44 ("x86/time: switch platform timer
hooks to altcall") did and eliminate the "real" read_tsc() altogether:
It's not used except in pointer comparisons, and hence it looks overall
more safe to simply poison plt_tsc's read_counter hook.
Signed-off-by: Jan Beulich <jbeulich@xxxxxxxx>
---
I wasn't really sure whether it would be better to use simply void * for
the type of the expression, resulting in an undesirable data -> function
pointer conversion, but making it impossible to mistakenly try and call
the (fake) function directly.
---
v2: Comment wording.
--- a/xen/arch/x86/time.c
+++ b/xen/arch/x86/time.c
@@ -607,10 +607,12 @@ static s64 __init cf_check init_tsc(stru
return ret;
}
-static uint64_t __init cf_check read_tsc(void)
-{
- return rdtsc_ordered();
-}
+/*
+ * plt_tsc's read_counter hook is not (and should not be) invoked via the
+ * struct field. To avoid carrying an unused, indirectly reachable function,
+ * poison the field with an easily identifiable non-canonical pointer.
+ */
+#define read_tsc ((uint64_t(*)(void))0x75C75C75C75C75C0ul)
static struct platform_timesource __initdata_cf_clobber plt_tsc =
{
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